Graphical representation of data in MATLAB

Tools for plotting

• plot- The plot and stem functions can take a large number of arguments, see help plot and help stem. For example the line type and color can easily be changed. plot(y) plots the values in vector yversus their index. plot(x,y) plots the values in vector yversus x. The plot function produces a piecewise linear graph between its data values. With enough data points it looks continuous.
• stem- Using stem(y)the data sequence yis plotted as stems from the x-axis terminated with circles for the data values. stem is the natural way of plotting sequences. stem(x,y) plots the data sequence y at the values specified in x.
• xlabel('string')- Labels the x-axis with string.
• ylabel('string')- Labels the y-axis with string.
• title('string')- Gives the plot the title string.

Example 1 

In this example we plot the function y = x2 for x 2 [-2; 2].

x = -2:0.2:2;

y = x.^2;

figure(1);

plot(x,y);

xlabel('x');

ylabel('y=x^2');

title('Simple plot');

figure(2);

stem(x,y);

xlabel('x');

ylabel('y=x^2');

title('Simple stem plot');

This code produces the following two figures.

Figure 1:

Figure 2:

• hold on / off - Normally hold is off. This means that the plot command replaces the current plot with the new one. To add a new plot to an existing graph use hold on. If you want to overwrite the current plot again, use hold off.
• legend('plot1','plot2',...,'plot N')- The legend command provides an easy way to identify individual plots when there are more than one per figure. A legend box will be added with strings matched to the plots.
• axis([xmin xmax ymin ymax])- Use the axis command to set the axis as you wish. Use axis on/off to toggle the axis on and off respectively.
• subplot(m,n,p) -Divides the figure window into m rows, n columns and selects the pp'th subplot as the current plot, e.g subplot(2,1,1) divides the figure in two and selects the upper part. subplot(2,1,2) selects the lower part.
• grid on/off - This command adds or removes a rectangular grid to your plot.

Example 2 

This example illustrates hold, legend and axis.

x = -3:0.1:3; y1 = -x.^2; y2 = x.^2;

figure(1);

plot(x,y1);

hold on;

plot(x,y2,'--');

hold off;

xlabel('x');

ylabel('y_1=-x^2 and y_2=x^2');

legend('y_1=-x^2','y_2=x^2');

figure(2);

plot(x,y1);

hold on;

plot(x,y2,'--');

hold off;

xlabel('x');

ylabel('y_1=-x^2 and y_2=x^2');

legend('y_1=-x^2','y_2=x^2');

axis([-1 1 -10 10]);

The result is shown below.

Subfigure 3.1 Subfigure 3.2 Figure 3

Example 3 

In this example we illustrate subplot and grid.

x = -3:0.2:3; y1 = -x.^2; y2 = x.^2;

subplot(2,1,1);

plot(x,y1);

xlabel('x'); ylabel('y_1=-x^2');

grid on;

subplot(2,1,2);

plot(x,y2);

xlabel('x');

ylabel('y_2=x^2');

Now, the result is shown below.

Figure 4:

Printing and exporting graphics

3D Graphics

Example 4 

Example: Creating our first 3D plot.

[x,y] = meshgrid(-3:.1:3);

z = x.^2+y.^2;

mesh(x,y,z);

xlabel('x');

ylabel('y');

zlabel('z=x^2+y^2');

This code gives us the following 3D plot.

Figure 5: